the 4 channels being bridged is 4 ohms stable the sub channel can go to 2 ohms the one I did in california is still running fine at 2 ohms.
Right on alpines website
Alpine Electronics of America, Inc.
Overview
Class-D
1/2 Channel Operation
Stackable Installation Design
RMS Power (at 14.4V THD+N, 20-20kHz)
Per channel into 4 ohms: 75W x 4 (0.08% THD)
Mono channel into 2 ohms: 300W (20-200Hz)